1. Field of the Invention
The present invention relates to a linear motor, and in particular to a control mechanism of a linear motor.
2. Description of the Related Art
In recent years, linear motors are more and more often used in various fields. For example, in a machine tool, linear motors are becoming increasingly popular for increasing feed rate in order to shorten machining time and for removing backlash and bending error due to a ball screw in order to improve machining precision. FIG. 3 shows an example structure of a portion of a machine tool which uses a linear motor. A bed 10 in FIG. 3 is a fixed portion of the tool and a stator 1 of a linear motor is fixed on the bed 10. A table 11 is mounted on the bed 10 via a linear guide or the like which is not shown, and is movable along the horizontal direction of FIG. 3. A slider 2 of the linear motor is fixed at a lower portion of the table 11 so that thrust is generated between the slider 2 and the stator 1, and the table 11 is driven. Normally, in a machining center, a machining object (or “workpiece”) is placed on an upper surface of the table 11, and the movement position of the machining object is controlled according to a machining program or the like to machine the machining object into a predetermined shape using a tool, which is not shown.
In these machine tools which use linear motors, because the feed rate is high, a braking distance during emergency stopping of the feed in the case of control abnormality, for example, tends to be long. Because of this, when an abnormality occurs during high speed movement at a portion near a stroke end of the feed mechanism, there are cases in which accidents occur such that the movable portion collides with the fixed portion of the tool, and the tool structure is damaged.
In order to prevent such accidents, machine tools of related art which use liner motors are typically designed to weaken the impact due to the collision by providing a shock absorber 12 at a stroke end, as shown in FIG. 3. Examples of the shock absorbers 12 include a shock absorber using a fluid resistance of a hydraulic fluid sealed inside the shock absorber, and a shock absorber which uses elastic deformation of a resin material such as rubber.
There is another general method as shown in FIG. 4 in which a limit switch 13 is provided at the stroke end and the limit switch is mechanically activated when the movable portion moves over an effective movable range. When the limit switch is activated, a contact 14 blocks a current to be supplied to a driving winding using a circuit, and at the same time, a contact 15 short-circuits the driving winding so that a dynamic brake is activated and the slider 2 is braked and stopped. The contacts 14 and 15 are formed using relays and magnet conductors, and are controlled to be opened and closed by a coil 16 which is connected in series to the limit switch 13. An inverter 17 supplies a current to a driving winding which is built into the slider 2.
In addition, there also is a typical method in which a braking mechanism using friction is provided along with the linear motor. By activating the braking mechanism with a limit switch as described above, it is possible to automatically brake and stop the movable portion when the movable portion moves over the effective movable range.
These methods and devices in the related art are described in, for example, Japanese Patent Laid-Open Publication No. Hei 9-151048 and Japanese Patent Laid-Open Publication No. Hei 8-251904.
When a shock absorber is used in a machine tool which uses a linear motor in order to prevent collision of the movable portion of the tool with the fixed portion, a large shock absorber having a sufficient braking capability must be used, and thus the size of the tool structure becomes large. In addition, there also is a problem in an increase in the cost if the shock absorber itself.
In a method as shown in FIG. 4 in which the driving winding is short-circuited to activate a dynamic brake, the operation delay time of the relay or the like which is a part of the contact 15 may be problematic. Specifically, the braking force is not generated until the contact 15 is switched ON, and thus the movable portion continues to move by inertia. In addition, there are also problems with the reliability of the operation of the contacts 14 and 15. When the conductive portions of the contacts are worn due to long-term usage, sufficient braking force cannot be achieved.
Moreover, in a structure using a braking mechanism using friction, the friction of the braking mechanism may be problematic. That is, when a contact surface of the brake is worn because of long-term usage, sufficient braking force cannot be achieved.
An object of the present invention is to realize, as a structure for preventing collision of a movable portion with a stroke end in a linear motor due to control abnormality or the like, a braking function having a simple structure, low cost, and a high reliability in which a wearing portion such as a relay contact and brake pad is eliminated.